This application claims priority to an application entitled xe2x80x9cPacket Data Communication Device and Method in Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Aug. 26, 1998 and assigned Serial No. 98-35310, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a communication method in a mobile communication system, and in particular, to a method of communicating packet data between a base station and a mobile station, which maximizes channel access success probability and minimizes channel access failure probability and dynamically changes the transmission rate.
2. Description of the Related Art
The conventional mobile communication system uses an offset period channel access and a channel assignment message loaded in one radio frame in order to achieve random access delay. On the assigned signal channel, service quality information needed for providing the multimedia service is exchanged, and a traffic channel is assigned to accommodate this.
FIG. 1 illustrates the structure of a broadcasting channel (BCCH) on which a base station provides system information. This system information is used by mobile stations requesting access from the base station in a conventional mobile communication system.
The BCCH includes a pair of radio frames, each radio frame being 10 ms in duration. BI identifies a radio frame, TX POWER indicates a transmission power level, and SFN represents for a superframe number. 72 radio frames are grouped into one superframe and used for calculation of the phase of a reverse long code. UP INTERFERENCE denotes the amount of reverse interference, W indicates the start, continuation, and end of an upper frame for each frame unit of a physical layer, CRC is used to detects errors in each MAC (Medium Access Control) sublayer frame, and TA is tail bits for convolutional coding.
FIG. 2 illustrates the structure of a conventional random access channel frame structure. This channel is used by mobile stations to request assignment of a dedicated traffic channel or to transmit a small amount of user packet data.
Referring to FIG. 2, D indicates a dummy bit, U/C (User Traffic/Control Traffic) Field indicates whether the signal is a request for dedicated traffic channel assignment or transmission of small user packet data, TN is a termination node indicating whether the corresponding data is to be processed in a base transceiver sysem or a base station controller, S is a sequence number used to determined whether input frame data has already been received, and PID is a paging identifier (ID) which distinguishes a mobile station.
FIG. 3 illustrates the structure of a forward access channel frame on which a base station transmits a response message to a mobile station upon access by the mobile station in the conventional mobile communication system. In FIG. 3, PID is the ID of the mobile station whose access request is received in the base station.
FIG. 4 illustrates a conventional access procedure of a mobile station using the channels shown in FIGS. 1, 2, and 3.
Referring to FIG. 4, the mobile station synchronizes its timing to a base station, acquires the offset of the random access channel (RACH) to be accessed, and attempts to transmit signal traffic. Then, if no other mobile station attempts to transmit at the same offset, the base station receives the data on the random access channel, and transmits an acknowledgment on the next forward access channel (FACH). If the mobile station has not received an acknowledgement after a predetermined time period, the mobile station retries access in the 16th slot.
The next-generation mobile radio communication system should ensure efficient performance of the random access method by the mobile station. To do so, the mobile radio communication system is required to minimize the time taken to receive an acknowledgment for an access request by reducing the access unit time, such as a time slot or a radio frame, and the amount of information transmitted during an access. An example is DSA++ (Dynamic Slot Assignment), a MAC protocol in which a channel access is attempted in mini-slots. Further, it is necessary to support as many random access channels as possible to minimize contention caused by concurrent access from a plurality of mobile stations.
Therefore, the channel access mechanism in the MAC protocol for radio multimedia service needs to increase the successful channel access rate by adding a reservation scheme to the conventional contention-based transmission scheme.
Further, to efficiently support the various Quality of Service (QoS) parameters required for multimedia traffic, radio channels should be dynamically managed. That is, a variety of traffics should be supported, such as CBR (Constant Bit Rate), RT-VBR (Real Time-Variable Bit Rate), VBR, and ABR (Available Bit Rate) over an ATM (Asynchronous Transfer Mode) network. Since dynamically varied traffic such as VBR should be supported in real time, a MAC sublayer must efficiently manage radio resources without signaling overhead.
The conventional mobile communication system cannot support multimedia traffic with a real-time-varying transmission rate because signal traffic must also be exchanged between layer-3 entities when a channel is to be additionally assigned during a service.
A channel access mechanism in a MAC protocol for radio multimedia service should support both contention-based transmission and reservation-based transmission. The conventional mobile communication system uses a slotted-ALOHA mechanism to implement channel access. A mobile station competitively requests assignment of a random access channel, and a base station assigns the channel when the base station acknowledges the access. In this case, access performance for supporting radio multimedia service is determined by the number of random access channels and the number of signal messages transmitted. To reduce delay in random access transmission, the conventional mobile communication system uses a channel access of an offset period and a channel assignment message which a single radio frame can accommodate. On the assigned signal channel, information about the QoS required to provide the multimedia service is exchanged and a dedicated traffic channel which can accommodate it is assigned. However, an additional exchange of signal messages is required to assign a new channel in a line mode service on a dedicated traffic channel in the conventional mobile communication system. This channel assignment and release procedure is difficult to implement in real time because it is executed by a layer-3 entity such as Radio Bearer Control (RBC).
It is, therefore, an object of the present invention to provide a method of assigning a channel in consideration of service quality and minimizing access contentions in a MAC sublayer in order to support multimedia service having various traffic characteristics in a mobile communication system which provides a packet data service.
It is another object of the present invention to provide a method of dynamically controlling a transmission rate according to the amount of transmission data during a service of a mobile communication system which provides a packet data service.
It is a further object of the present invention to provide a method of transmitting status information required for random access on a specific channel by a base station in a mobile communication system.
It is still another object of the present invention to provide a device and method for minimizing contention during channel access by a mobil station in a mobil communication system.
It is yet another object of the present invention to provide a device and method for dynamically controlling the transmission rate depending on the amount of data transmitted during data transmission in a mobile communication system.
To achieve these and other objects, there is provided a method for controlling channel access in a mobile communication system. A base station generates a broadcast channel frame, which includes status information indicating whether continuously changing channel codes are occupied or not, and transmits it at a predetermined interval. Then, a mobile station selects an available channel code based on information from the received broadcast channel frame, generates a channel assignment request message, and transmits the channel assignment request message on a random access channel. Upon reception of the channel assignment request message on the random access channel, the base station assigns a channel, sets a transmission rate, and transmits the information on a forward access channel.